Nearly 400,000 Rohingyas have fled to Bangladesh to escape a military offensive that has been described as ethnic cleansing, and raised fears of an unfolding humanitarian crisis.

U.S. Deputy Assistant Secretary of State Patrick Murphy was due in Myanmar this weekend to voice Washington's concerns and press for greater access to the conflict area for humanitarian workers, the State Department said.

Read: Suu Kyi Says Myanmar Trying to Protect All Citizens

"We don’t block anyone," Myanmar's government spokesman Zaw Htay told Reuters.

"We don’t block any organisations sending aid to those areas but they might have some difficulty travelling where access is restricted by local authorities for security reasons."

It was unclear whether Murphy would be visiting Rakhine State, and Zaw Htay declined to say whether any request by the U.S. diplomat would be accepted.

The military offensive was launched in response to coordinated attacks on police posts and an army camp by fighters from the Arakan Rohingya Salvation Army (ARSA) that killed a dozen people.

The crisis has raised questions about Buddhist-majority Myanmar's transition to civilian rule under the leadership of Nobel laureate Aung San Suu Kyi following decades of military rule.

The generals still control national security policy but, nevertheless, Suu Kyi has been widely criticised abroad for not condemning the violence. The campaign against the Muslim insurgents is popular at home.

U.S. Senate Majority Leader Mitch McConnell said on Thursday he had spoken with Suu Kyi and that she said she was working to get aid to areas affected by violence.

REUTERS